The unique combination of characteristics of cacti clearly make them a distinct family, but these same features indicate relationships to other plants. One of the most distinctive features of cacti is that they contain pigments including the nitrogen-containing betalains (water-soluble pigments, including betacyanins and betaxanthins) rather than the non-nitrogen-containing antho-cyanins (water-soluble flavonoids) typical of most other flowering plants. This is strong evidence that the family belongs in the betalain-con-taining order Caryophyllales.
The family Cactaceae is also characterized by possessing large numbers of alkaloids, nitrogen-containing compounds. Lophophora williamsii, for example, is known to have more than 50 different alkaloids (Anderson 1996, 220-228) with both phenethylamines and tetrahydroisoquino-lines present. Triterpenes and sterols, steroids containing several carbon rings and alcohol side chains, also occur in the family.
Mucilage is characteristic of many groups of cacti, probably functioning in water storage (Gibson and Nobel 1986, 194-199). Ariocarpus and Opuntia are two genera that develop complex mucilage systems. Some species of the genus Mam-millaria also possess latex-producing cells or lati-cifers that arise from the breakdown of numerous cells, producing wide ducts in the cortex (Mauseth 1978a,b, 1983a; Wittier and Mauseth 1984).
Many cacti have idioblastic cells, cells that differ in size, shape, or function from those in the surrounding tissue. Druses, aggregates of crystals of calcium oxalate, are located within the hypo-dermis and cortex. Their presence, location, and shape are useful taxonomic characters, as shown in Pelecyphora (Anderson and Boke 1969). Silica bodies are found in all species of Stenocereus (Gibson and Nobel 1986,203-204).
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